Helium is the second-smallest and second-lightest element in the universe, behind hydrogen. But unlike hydrogen, it’s not flammable — which is why lighter-than-air craft have opted for helium ever since the Hindenburg disaster spelled the end of the hydrogen-filled blimp. While there aren’t very many blimps around today, they’re used in tourism, advertising and, of course sports coverage — and are playing an increasingly valuable role in military surveillance, where soldiers in forward operating bases use them to spot enemies hiding just over the horizon.